Spewing
from factory stacks and car tail pipes,carbon
dioxide is the
poster child of "greenhouse gases." Most
scientists long ago concluded that CO2 is
the single biggest cause of climate change
and that cutting its output is the best
way to slow global warming and suffocation
by all life forms.

So why are a tiny but growing number of
atmospheric scientists taking a hard look
at parking lots? Because, they
say, land-use changes have at least as
much, and perhaps even greater, impact
on climate change than CO2. It's a
radical idea that has heated up the
scientific community and is prompting a
wider look at the forces behind climate
change. The effect on public policy could
be enormous.

Do massive asphalt and concrete "urban
heat islands" like Houston or Atlanta
really help ratchet up the global
thermostat? What about huge tracts of
farmland like those that span the
Midwest?

Eugenia Kalnay thinks so. Her research
into the impact of land-use changes on
global temperature is getting attention
from other scientists, even if this debate
hasn't exactly leaped into the public
arena yet.

Earth's surface temperatures have risen
about 1 degree F. in the past century with
faster warming in the past two decades,
the National Academy of Sciences reports.
The 20th century's 10 warmest years all
occurred in the last 15 years of the
century.

But according to Dr. Kalnay's study,
published in the journal Nature last
spring, urbanization, agriculture, and
other human changes to landscapes in the
US - quite aside from CO2 - account for as
much as 40 percent of the temperature rise
over the past 40 years - much larger than
previously believed. That could make it a
contender for CO2's crown.

Kalnay, a University of Maryland
researcher, was director of environmental
modeling at the National Weather Service
from 1987 to 1997. She oversaw development
of computer models for the now ubiquitous
three- to five-day forecasts.

But it is her recent research that struck
a chord with the scientific community.
Kalnay and coauthor Ming Cai have received
a huge amount of both praise and
criticism. "We were both taken aback that
instead of the paper going quietly, we got
hundreds and hundreds of comments and
questions," she says.

Now Kalnay's research, joined by the work
of a growing number of other scientists,
has intensified debate over the relative
strength of "climate forcing"
factors.

Recent studies show that deforestation in
parts of Africa is curbing rainfall in
the once-vital Sahel zone bordering
the Sahara desert. Changes in forest cover
have also been shown to affect rainfall
and climate far beyond the Amazon Basin.
Still others have shown that planting
trees can actually increase the planet's
temperature if done in the wrong climate
zones.

"Impacts of human-caused land changes on
climate are at least as important, and
possibly even more important, than those
of carbon dioxide," says Roger Pielke Sr.,
professor of atmospheric science at
Colorado State University and past
president of the American Association of
State Climatologists. His group voted in
2002 to issue a statement almost
unanimously concurring that climate
changes are more complex than CO2 changes
and include land use. By contrast, the
American Geophysical Union issued a
statement last month maintaining CO2 as
the key factor.

Dr. Pielke and others argue that land-use
changes in a region may have significant
effects thousands of miles away - not
unlike the El NiŮo effect in which warming
zones of the Pacific Ocean force droughts
and weather changes worldwide.

That's still theory, of course. Skeptics
point out that only 29 percent of the
earth's surface is land - and only 1 to 2
percent is urbanized. Another 40 percent
of land has been modified by agriculture
and deforestation, Pielke says. So can the
regional land-use tail really wag the
global dog?

Not according to Alan Robock, an editor of
the Journal of Geophysical Research:
Atmospheres. He's also seen no uptick in
studies of land-use impact on climate in
scientific literature he's read.

Still, the view that human changes to the
landscape are a factor driving climate,
too, is gaining some traction in powerful
corners of the scientific community. A
report by the National Academy of Sciences
due later this year will examine the
warming effects of non-CO2 agents:
aerosols, solar variability, and land-use
changes.

"The public does not hear too much about
this, because all the climate-change
treaties have been focused on CO2," says
Daniel Jacob, a Harvard University
professor of atmospheric chemistry who
chairs the panel writing the report. "For
a long time it's been really hard to
communicate these other factors to the
policymakers, mainly because it's
difficult to find the proper currency for
them."

The impact of such change would begin
first with global climate modelers, like
Robert Dickinson, president of the
American Geophysical Union. Dr. Dickinson
is working to include more detailed
effects from land-surface changes,
aerosol, and soot in his climate model. He
says one of his graduate students is
pursuing a surface-temperature study of
China. But like many, he still maintains
that CO2 is the dominant force in climate
change.

Kalnay's research is providing ammunition
for some private groups to argue that
global warming is a myth. In an editorial
last June, the Center for the Study of
Carbon Dioxide and Global Change argued
that Kalnay's work showed that the impact
of CO2 was overstated. "The warming of the
past century or so was nothing more nor
less than the natural recovery of the
earth from the global chill of the Little
Ice Age," the Tempe, Ariz., nonprofit
reported.

Such conclusions irk Peter Frumhoff,
global environment program director for
the Union of Concerned Scientists in
Cambridge, Mass. "Just because new
research shows there are other factors to
pay attention to [beside CO2, that]
doesn't mean there's any less reason to
pay attention to greenhouse gases," he
says.

Kalnay is undeterred. Having completed her
temperature study of the US, she is
working on a global analysis of 50 years
of temperature data. Already, early
results from South America support her
conclusions. "Greenhouse gases are
undoubtedly very important," she says. "But
the second cause for climate change is
the way we are using the land surface."

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